This invention relates to an ink-jet printing apparatus and method and, more particularly, to an inkjet printing apparatus in which a carriage mounting an ink-jet printhead having a plurality of print elements arrayed in at least two rows for performing printing by discharging ink from the print elements is scanned back and forth above a print medium in a direction cross to the direction in which the print elements are arrayed to thereby print on the print medium, and to a printing method using this apparatus.
A printer for recording information such as desired characters and images on a sheet-like print medium such as paper or film is available as an information output apparatus in a word processor, personal computer and facsimile machine, etc.
Though a variety of printing methods implemented by such a printer are known, the ink-jet method in particular has been the focus of attention in recent years because it is capable of printing on a print medium such as printing paper without making mechanical contact with the medium, because color printing can be performed with ease and because printing can be performed silently. From the viewpoint of low cost and small size, a widely employed printing technique is serial printing in which the ink-jet printing apparatus is fitted with a printhead for discharging ink in accordance with the desired print information and printing is carried out while the printhead is scanned back and forth at right angles to the direction in which the print medium such as printing paper is fed.
Recent improvements in the performance of such ink-jet printers have made it possible to achieve a high printing speed on a par with that of laser printers. An increase in the processing speed of personal computers and the spread of the Internet have led to ever greater demand for the printing of color images at higher and higher speeds.
One ink-jet technique is the bubble jet printing technique in which ink droplets are discharged from orifices by the pressure of bubbles produced by rapidly heating and vaporizing ink by means of a heating element. A bubble produced in a bubble jet printhead having such a structure is cooled by the surrounding ink so that the ink vapor within the bubble is condensed back into liquid form and vanishes.
Ink expended by discharge is replenished via an ink supply path from an ink tank containing the ink.
FIG. 9 is a diagram illustrating an example of the arrangement of nozzles 4 in such an ink-jet printhead. A row of even-numbered nozzles 4 is disposed on the left side of an ink supply path 105 and a row of odd-numbered nozzles 4 is disposed on the right side of the ink supply path 105. Ink is supplied from the ink supply path 105 via an ink flow path 5 provided in correspondence with each nozzle 4.
A known ink-jet printing technique other than the above-described bubble jet printing technique is the bubble through jet printing technique through which bubbles produced by rapidly heating and vaporizing ink by means of a heating element are discharged upon being passed through the outside air.
FIGS. 3A to 3D and FIGS. 3E to 3H illustrate the manner in which droplets are discharged in accordance with the bubble jet and bubble through jet techniques, respectively. The shape of the discharged ink varies in a time series as indicated by FIGS. 3A to 3D and FIGS. 3E to 3H.
Ink that has been discharged from the nozzle 4 leaves a tail immediately after being discharged, as in FIGS. 3A and 3E. A main droplet then starts to form at the leading edge in FIGS. 3B and 3F, the ink splits into a main droplet and satellites, as shown in FIGS. 3C and 3G, at the moment surface tension attempting to coalesce the ink is overcome, and the ink finally splits into a main droplet 31 and satellites 32, as indicated in FIGS. 3D and 3H.
The printhead which discharges ink by the above-described techniques will be described with reference to FIGS. 4A to 4C in regard to a case where the printhead is mounted on a transported carriage and a printing operation is carried out by moving the carriage back and forth in a direction cross to the direction in which the print medium is transported.
As illustrated in FIG. 4A, an ink droplet discharged from each ink nozzle of an even-numbered nozzle row and an ink droplet discharge from each nozzle of an odd-numbered nozzle row of each chip of a printhead IJH comprises a main droplet 78i (i =1xcx9cn), which impacts upon a print surface 76a of paper 76 serving as an example of a print medium to thereby form the main portion of one pixel constituting an image or character, and a satellite 78ai (i =1xcx9cn), which is the remainder of the discharged main droplet 78i. The satellite 88ai flies is a fixed direction in dependence upon the release characteristics of the ink droplet, e.g., the printhead characteristics (the shape of the ink nozzle and the properties of the ink fluid).
In a case where the printhead IJH travels in the direction indicated by arrow F in FIG. 4A, e.g., in the forward direction, the impact position of the satellite 78ai relative to the main droplet 78i will be on the side opposite that of the direction of travel when the flying direction of the satellite 78ai, which has been discharged from the nozzle of the row of even-numbered nozzles, relative to the main droplet 78i is opposite the direction indicated by arrow F.
Accordingly, if the structure of the printhead is such that the arrays of the nozzles on the sides of the even and odd numbers are substantially symmetrical with respect to the centrally located ink supply path, as illustrated in FIG. 9, the direction in which the satellite 78ai from the nozzle of the odd-numbered nozzle row flies relative to the main droplet 78i will be diagonally away from the main droplet 78i in a direction the same as that indicated by arrow F, so that the impact position of the satellite 78ai relative to the main droplet 78i will be on the side of the direction of travel.
As a result, a portion 82ai formed by the satellite 78ai discharged from the nozzle of the even-numbered nozzle row is formed so as to overlap and become included within a portion 80i formed by the main droplet 78i that has impacted the printing surface 76a, as depicted in FIG. 4B. On the other hand, a portion 82ai formed by the satellite 78ai discharged from the nozzle of the odd-numbered nozzle row is formed outside the portion 80i formed by the main droplet 78i that has impacted the printing surface 76a, as depicted in FIG. 4C.
Further, in a case where the printhead IJH travels in the direction indicated by arrow R in FIG. 4A, e.g., in the reverse direction, the portion 82ai formed by the satellite 78ai discharged from the nozzle of the even-numbered nozzle row is formed outside the portion 80i, as shown in FIG. 4C. On the other hand, the portion 82ai formed by the satellite 78ai discharged from the nozzle of the odd-numbered nozzle row is formed so as to overlap and become included within a portion 80i formed by the main droplet 78i that has impacted the printing surface 76a, as depicted in FIG. 4B.
If the printhead prints a character or the like on a printing surface by being moved in the forward and reverse directions, the character is formed mostly by the main droplets 78i, and the satellites 78ai are mostly covered by the main droplets. However, the portion 82ai formed as shown in FIG. 4C at the edge portion of a printed character by a satellite not covered by the portion 80i formed by the main droplet will have an influence upon print quality. This portion appears as edge roughness at several parts of the printed character.
FIGS. 5A and 5B illustrate examples of edge roughness when characters are printed, in which FIG. 5A represents roughness due to printing in the forward direction and FIG. 5B roughness due to printing in the reverse direction. Such edge roughness of a character is noticeable visually and detracts from print quality.
Accordingly, an object of the present invention is to provide an ink-jet printing apparatus and method through which print quality can be improved by reducing edge roughness.
According to the present invention, the foregoing object is attained by providing an ink-jet printing apparatus in which a carriage mounting an ink-jet printhead having a plurality of print elements arrayed in at least two rows for performing printing by discharging ink from the print elements is scanned back and forth over a print medium in a direction cross (preferably, substantially orthogonal) to the direction in which the print elements are arrayed to thereby print on the print medium, the apparatus comprising: print data generating means for generating print data, which is in conformity with the structure of the printhead, from entered information; thinning means for thinning, from the print data generated, and in dependence upon the scanning direction of the carriage, print data printed by print elements of at least one of the two rows thereof; and driving means for driving the printhead in accordance with the print data that has been thinned by the thinning means.
Preferably, the thinning means alternately thins 50% or 100% of print data printed by the print elements of one row.
In a case where the entered information includes a character, edge detection means is provided for detecting print data representing an edge portion of the character, and the thinning means thins the print data representing the edge portion from the print data printed by the one row of print elements.
Further, according to the present invention, the foregoing object is attained by providing an ink-jet printing method in which a carriage mounting an ink-jet printhead having a plurality of print elements arrayed in at least two rows for performing printing by discharging ink from the print elements is scanned back and forth over a print medium in a direction cross (preferably, substantially orthogonal) to the direction in which the print elements are arrayed to thereby print on the print medium, the method comprising: a print data generating step of generating print data, which is in conformity with the structure of the printhead, from entered information; a thinning step of thinning, from the print data generated, and in dependence upon the scanning direction of the carriage, print data printed by print elements of at least one of the two rows thereof; and a driving step of driving the printhead in accordance with the print data that has been thinned at the thinning step.
Thus, in an ink-jet printing apparatus in which a carriage mounting an ink-jet printhead having a plurality of print elements arrayed in at least two rows for performing printing by discharging ink the print elements is scanned back and forth above a print medium in a direction cross to the direction in which the print elements are arrayed to thereby print on the print medium, print data conforming to the structure of the printhead is generated from entered information, print data printed by print elements of at least one of the two rows is thinned from the print data in conformity with the scanning direction of the carriage, and printing is performed by driving the printhead in accordance with the thinned print data.
Thus, the arrangement is such that if a satellite droplet associated with a main droplet of discharged ink is released at a certain angle, a noise component referred to as xe2x80x9cedge roughnessxe2x80x9d is produced at an edge portion as the result of a satellite droplet impacting externally of a pixel area of the main droplet when either of the rows of print elements is driven in conformity with the scanning direction of the carriage. However, the print data that gives rise to this noise component is thinned so that the edge portion of the printed image can be sharpened.
Accordingly, a high-quality image having a sharp edge is obtained. This has the effect of improving print quality because edge portions are noticeable especially when printing a character. Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.